1. Field of the Invention
This invention relates to the field of hydraulic systems, more particularly it pertains to control of speed ratio changes produced by such systems in an automatic transmission for a motor vehicle.
2. Description of the Prior Art
Shifting a synchronous automatic transmission is accomplished by applying or releasing a friction element (band brake or clutch) that changes the speed and torque relationship by means of a planetary gearset. Precise control of friction element torque capacity is vital to providing and maintaining good shift quality and durability. Because attaching a sensor directly to the friction element actuator can be difficult or impossible, conventional synchronous shift control strategies depend on estimating actuator pressure to infer actuator position and friction element torque capacity. Unit-to-unit variations and component variations over the life of the transmission may cause shift quality degradation with the conventional approach. In addition, response of software control to driver change-of-mind gearshifts relies on estimated clutch capacity with the conventional approach, which may not be optimum in all situations because of uncertainty in the estimate.